Treatment of spinal injuries or disorders may include the use of a bone fixation device, such as a relatively rigid fixation rod and/or fixation plate coupled to spinal features on opposite sides of a site of injury or disorder to facilitate healing or correction of the injury or disorder. For example, it is known to attach a bone fixation plate to adjacent vertebrae to provide increased rigidity between the adjacent vertebrae in the case of a discectomy or a degenerative condition, in order to relieve pain and/or facilitate healing and/or fusion between the adjacent vertebrae.
A bone fixation plate is typically fixed across a fracture, damaged or removed disc, or other location of injury or disorder, by screwing opposite ends of the plate to healthy tissues, typically bone, on opposite sides of the location of injury or disorder, thereby rigidifying the features to which the plate is mounted to, as well as the location therebetween. In addition to providing the ability to fix the features joined by such a bone plate, it would be desirable to provide some additional, structural support, while retaining the ability of some limited relative motion between the joined features. This is particularly the case in some spinal surgeries, where it is necessary to provide some support to the columnar strength of the spinal column, but where it would also be desirable to allow some limited range of motion between vertebrae joined by a plate. The option to convert such a plate from one that completely rigidifies the vertebrae attached, relative to one another, to one that provides some rigidity and limits motion between those vertebrae, but allows a limited range of motion, and vice versa, would also be desirable.
It would further be desirable to provide a device useful not only for providing a first time solution for spinal repair, but which is additionally useful for revision surgery, or one that can span several (at least up to four) levels of the spine.
It is also common, especially in vertebral surgeries of the type described above to experience subsidence of the vertebrae after fixation of the plate thereto, such as during healing (e.g., which fusion is developing). Bone plate systems that do not allow any movement after fixation can experience failures such as partial or total pull out of one or more screws, shearing of one or more screws, or other failures induced by the compressive forces on the bone plate system caused by the subsidence. Accordingly, it would be beneficial to provide solutions that allow subsidence to occur without significantly increasing or distorting stresses imposed on the bone plate system thereby.
On the other hand, it would further be desirable to prevent or reduce the ability of bone portions fixed by a bone plate to extend away from one another, as this can delay or prevent the desired healing, and can also be painful.
Another problem with bone plate systems in general is that screws used to fix the plate to features of the patient can loosen over time, after implantation, and actually “back out” to a lesser or greater degree from their implanted positions, by reverse rotation along their thread paths. It would further be desirable to provide devices having one or more of the desirable features described above, which additionally are provided with a backout prevention mechanism to prevent such backout.